What processes explain the genetic and morphological variation in this neotropical species?
Speciation involves the build-up of genetic differences between populations in isolation. But this “isolation” can take many forms. Populations might be separated over large distances, preventing the exchange of genetic material (isolation by distance). Or populations might become isolated by historical processes that affect topographic features, such as rivers and mountain ranges (isolation by history). Or populations might adapt to different environmental conditions in a heterogenous landscape, lowering the chances of interbreeding (isolation by environment). These types of isolation are not mutually exclusive and can interact. Not an easy knot to disentangle. A recent study in the journal Molecular Ecology, however, rose up to the challenge and attempted to figure out what isolating factors can explain the evolution of the Altamira oriole (Icterus gularis).
The Altamira oriole is a brightly colored songbird from Central America. Taxonomists recognize three subspecies based on body size: the large gularis, the small mentalis, and the intermediate flavescens. Lucas Moreira and his colleagues studied populations from all subspecies, using molecular and morphological methods.
The genetic analyses revealed two distinct clusters, divided by the Chivela Pass, a narrow mountain gap separating three mountain chains in southern Mexico. The split between these populations occurred about 150,000 years ago, but the populations remained connected by occasional gene flow. It seems that the Chivela Pass functioned as a kind of funnel that limited dispersal between the northern and southern populations. Within both populations, the researchers detected signatures of isolation by distance, but no effect of environmental factors. Hence, the genetic data point to isolation by history and isolation by distance.
The morphological data tell a completely different story. Measurements of wing, bill and tarsus length indicated two main groups, representing small and large individuals. These differences in body size were best explained by climatic variables, such as precipitation and temperature. In general, Altamira orioles are larger in hotter and drier areas. Whether these morphological differences have a genetic basis or are due to phenotypic plasticity remains to be determined. Nonetheless, morphological variation can best be explained by isolation by environment.
The fact that genetic and morphological patterns are driven by different isolation mechanisms makes sense from a genomic point of view. Isolation by history and isolation by distance tend to affect the entire genome, and it is thus easier to pick up these patterns with molecular markers that cover the whole genome (RADseq in this study). The morphological differences are often the outcome of natural selection, which tends to target small genomic regions and is thus more difficult to detect without whole genome data. A genomic study of the Altamira oriole might thus uncover some contribution of isolation by history to the morphological differences.
In addition, the different processes underlying genetic and morphological variation also explain the disagreement in taxonomic studies where genetically focused ornithologists clash with morphologically minded ones. This study again highlights that it is more insightful to understand the evolutionary and ecological processes behind speciation instead of having endless discussions about species status based on every small genetic difference or morphological detail.
Moreira, L. R., Hernandez_Ba_os, B. E., & Smith, B. T. (2020). Spatial predictors of genomic and phenotypic variation differ in a lowland Middle American bird (Icterus gularis). Molecular Ecology, 29(16), 3084-3101.
Featured image: Altamira Oriole (Icterus gularis) © Kati Fleming | Wikimedia Commons